The present invention relates generally to the field of antennas and, more particularly, to the field of antennas that radiate in the 30-88 MHz frequency range.
A discone antenna is a vertically polarized, wideband antenna providing omnidirectional coverage in a simple mechanical design. The outer conductor of the transmission line associated with the antenna is connected to the cone radiating portion of the antenna at the gap between the cone and the disc radiating portion and the inner conductor of the transmission line is connected to the center of the disc. The disc and the cone are insulated from each other by suitable means. The feed point impedance of a discone antenna may be close to 50 ohms and result in good VSWR in the design frequency range. Over the frequency range 30-88 MHz, gain, on the horizon (0.degree.), is comparable to a ground plane or a biconical antenna.
Such discone antennas require antenna volumes which are about twice as large for the same low frequency cutoff point as the present invention, are too large for transportable field use at low VHF and high HF frequencies, are much more difficult to install and to remove than the present, require more maintenance effort and are more expensive to purchase than the antennas disclosed herein.
The United States Department of Defence (DoD) utilizes a ground plane antenna known as the RC-292 antenna which operates over a 30-76 MHz range by use of three physical configurations. The RC-292 antenna is illustrated in FIG. 1 and will be described further in the body of the specification herein. Subbands for these three physical configurations are 30-36.5, 36.5-50.5 and 50.5-76 MHz. As designed, the RC-292 is a ground-plane antenna and has limited VSWR bandwidth and gain. The operational requirements for this antenna often force the operators to use the midband configuration across the entire 30-76 MHz band and, as a result, efficiency is degraded to as little as 10% when compared to a properly configured RC-292 antenna. Further, it has recently become desirable to extend the frequency range of the RC-292 up to 88 MHz.